1. Field of the Invention
The invention relates in general to a sequential control circuit, and more particularly to a sequential control circuit, which asserts a plurality of control signals in a predetermined sequence and de-asserts the plurality of control signals in a sequence reverse to the predetermined sequence according to a state of an input signal.
2. Description of the Related Art
With the rapid development of the technology, the typical digital manipulation circuit can be easily designed and formed in the IC chip, and different IC chips have different special functions. Generally speaking, each IC chip has its specific operation mode or method. So, the IC chip manufacturers provide specifications to introduce the pins, the connections and the operations of the IC chips in order to avoid the damage or glitch of the IC chips due to the incorrect usage of the user.
Typically, the specifications of some specific IC chips, such as the LSI-8602 chip, specify the assertion sequence. The IC chip has to be turned on in a predetermined sequence to control the signals to be inputted to different pins, and to be turned off by de-asserting the inputs of the control signals in a sequence reverse to the predetermined sequence. For example, the IC chip can be normally turned on only when the high-level control signals are sequentially inputted to the pins A to D. The sequence cannot be changed or the IC chip may have glitch or error. Furthermore, the IC chip has to be turned off normally only when the low-level control signals are sequentially inputted to the pins D to A. The sequence cannot be changed or the IC chip may have glitch or error.
FIG. 1 is a schematic illustration showing an IC chip requiring the sequential assertion and the sequential de-assertion, and connection wires for the IC chip. In order to make the IC chip 6 operate normally and without error, the designer makes the IC chip 6 to be connected to a microprocessor (or micro controller) 4 and controls the firmware program to control the IC chip 6. As shown in FIG. 1, four input/output ports (I/O-1, I/O-2, I/O-3, I/O-4) of the microprocessor 4 are respectively connected to pins A to D of the IC chip 6. When the IC chip 6 wants to turn on, the firmware program enables the four input/output ports to output control signals 1000, 1100, 1110 and 1111 (1 represents the high level and 0 represents the low level) sequentially. Thus, the IC chip 6 can turn on normally according to the high-level control signals sequentially received by the pins A to D. On the contrary, when the IC chip 6 wants to turn off, the firmware program enables the four input/output ports to output the control signals of 1110, 1100, 1000 and 0000 sequentially. Thus, the IC chip 6 can turn off normally according to the low-level control signals sequentially received by the pins D to A.
However, because the number of pins of the input/output port of the microprocessor 4 is restricted, it is an important subject of the invention to assert the control signals in a predetermined sequence with a less number of input/output pins and low-cost circuit devices.